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IEEE Transactions on Magnetics

Dynamics of some single flux quantum devices: I. Parametric quantron

Konstantin K. Likharev

Published Jan 1, 1977• 117 citations
Publisher

Abstract

In this paper and the following one, the properties of two systems operating with single flux quanta are calculated. The first system is a quantron (a one-junction interferometer) with a Josephson gate as a junction. By a proper change of its critical current in time, the switching of quantron stable states can be controlled by a small external flux much less than Φo. This "Parametric Quantron" can be used as the basic cell of a Single Quantum Logic System. The most wonderful property of this device is that the energy dissipation during one logical step (PτT factor) can be much less than the thermal energy KBT and is limited by quantum effects only: Pτ2\simg102h.

DOI
10.1109/tmag.1977.1059351

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We have theoratically and experimentally investigated the possibility of the phase mode operation of symmetrical four-junction Josephson interferometers (4J-loop), which are considered to play a key role as quantum logic devices for a Josephson switching system utilizing single-quanta as information bits, because of their excellent characters such as the kinetic momentum quantum effect and variety of the logic operations. The threshold characteristic, which is the most basic for Josephson interferometers to be operated in the phase mode and shows their fluxoid-quantum states, has experimentally been observed clearly for the first time for symmetrical 4J-loop's by using a novel radial-scanning, intensity-modulation technique, as predicted theoretically. It is characterized by a limited closed-shaped pattern. The number of the fluxoid-quantum states increases with LI0for the loops. Two different basic types of switching behaviors, a vortex-to-vortex and a vortex-to-voltage transition, have been confirmed, which have clearly been separated on the threshold characteristic, for the loops with LI0larger than\sim0.7\Phi_{0}The optimum LI0value for the phase mode operation of the loops has been found to be0.75\Phi_{0}\lsim LI_{0}\lsim 1.25\Phi_{0}. The desired performance of the phase mode operation of symmetrical 4J-loop has been confirmed by the shift motion of single fluxoids on the basis of the vortex-to-vortex transition of the loop. Finally, an application of such phase mode Josephson interferometers for pulse generators of Josephson sampling systems has been proposed.

IEEE Transactions on Magnetics•Mar 1, 1985•6 citations
In this paper and the following one, the properties of two systems operating with single flux quanta are calculated. The first system is a quantron (a one-junction interferometer) with a Josephson gate as a junction. By a proper change of its critical current in time, the switching of quantron stable states can be controlled by a small external flux much less than Φo. This "Parametric Quantron" can be used as the basic cell of a Single Quantum Logic System. The most wonderful property of this device is that the energy dissipation during one logical step (PτT factor) can be much less than the thermal energy KBT and is limited by quantum effects only: Pτ2\simg102h.
Some of the behaviour of a quantron (one-junction interferometer) with a Josephson gate is simulated. This quantron was proposed by Likharev in 1977 and is called the Parametric Quantron. By proper control of its critical current with time, alternative stable states of the quantron can be selected by a switching signal whose product of the energy dissipation E c and the switching time τ is as low as 10 2 h (where h is Planck's constant). In this experiment, a simulator based on the analogy between a Josephson junction and a phase-locked loop is used. Analog systems are flexible and easier to assemble than the mechanical analog.
A new type of logic gate that can be designed using a nonhysteretic Josephson weak link is proposed. The basic component of the proposed device is a one-junction interferometer, and a logic state is represented by either a zero or a single-flux-quantum. In contrast to the "Parametric Quantron," this device is designed to operate without a three-phase clock and the dependence of the junction critical current on magnetic field is not used. The switching behavior of the device was simulated by computer and an analytical expression for the switching delay has been obtained.
A new family of dc-powered Josephson junction digital devices, the Rapid Single Flux Quantum (RSFQ) logic, is described. The devices use overdamped Josephson junctions and two-junction interferometers to store, pass and process the digital information presented in form of single flux quanta. We have carried out extensive numerical simulation of the dynamics of the RSFQ logic gates and of some more complex circuits including serial full adder and reversible shift register, within the standard microscopic-theory ("Werthamer") description of Josephson junctions. The minimum clock cycles of the basic RSFQ circuits turn out to be as small as 2.5 ps. The most promising ways to use the RSFQ logic circuits at the present stage of development of the Josephson junction digital technology are discussed.
Flux transfer device theory is reviewed. In such devices, generalized magnetic flux, defined as the time integral of voltage, is used to describe the device characteristics. The Josephson junction and inductor are the main circuit elements in flux transfer devices, because they maintain a constant value for the time integral of voltage. Flux transfer devices are based on either an RF SQUID (superconducting quantum interference device) or a fluxon device. Four devices, including the parametric quantron and the quantum flux parametron, are reviewed as applications of the RF SQUID. The fluxon feedback oscillator and a soliton device are also reviewed as applications of fluxon devices. The quantum flux parametron is then described. The parametron principle and the fundamental properties of the quantum flux parametron such as gain, switching speed, and power dissipation are discussed. Logic circuits and a memory cell are also reviewed. A novel analog-to-digital converter is proposed as an application of the quantum flux parametron.>
A new type of single flux quantum logic gate is proposed, which can perform unilateral propagation of signal without using three-phase clock. This gate is designed to be built with bridge-type Josephson junctions. A basic logic gate consists of two one-junction interferometers coupled by superconducting interconnecting lines, and the logical states are represented by zero or one quantized fluxoid in one of one-junction interferometers. The bias current of the unequal magnitude to each of the two one-junction interferometers results in unilateral signal flow. By adjusting design parameters such as the ratio of the critical current of Josephson junctions and the inductances, circuits with the noise immunity of greater than 50% with respect to the bias current have been designed. Three cascaded gates were modeled and simulated on a computer, and the unilateral signal flow was confirmed. The simulation also shows that a switching delay about 2 picoseconds is feasible.
We have theoratically and experimentally investigated the possibility of the phase mode operation of symmetrical four-junction Josephson interferometers (4J-loop), which are considered to play a key role as quantum logic devices for a Josephson switching system utilizing single-quanta as information bits, because of their excellent characters such as the kinetic momentum quantum effect and variety of the logic operations. The threshold characteristic, which is the most basic for Josephson interferometers to be operated in the phase mode and shows their fluxoid-quantum states, has experimentally been observed clearly for the first time for symmetrical 4J-loop's by using a novel radial-scanning, intensity-modulation technique, as predicted theoretically. It is characterized by a limited closed-shaped pattern. The number of the fluxoid-quantum states increases with LI0for the loops. Two different basic types of switching behaviors, a vortex-to-vortex and a vortex-to-voltage transition, have been confirmed, which have clearly been separated on the threshold characteristic, for the loops with LI0larger than\sim0.7\Phi_{0}The optimum LI0value for the phase mode operation of the loops has been found to be0.75\Phi_{0}\lsim LI_{0}\lsim 1.25\Phi_{0}. The desired performance of the phase mode operation of symmetrical 4J-loop has been confirmed by the shift motion of single fluxoids on the basis of the vortex-to-vortex transition of the loop. Finally, an application of such phase mode Josephson interferometers for pulse generators of Josephson sampling systems has been proposed.